In a conventional image forming apparatus, a latent image on an image carrier is developed by a toner supplied from a developing device so that a toner image is formed on the image carrier as a visual image. The toner image on the image carrier is transferred onto a transfer medium by a transferring device so as to be fixed on the transfer medium by a fixing device. The fixing device, for example, includes a heater, a fixing roller, and a pressure roller. The heater is provided inside of the fixing device as a heating source. The fixing roller works as a rotatable surface moving body. The pressure roller comes in contact with a surface of the fixing roller at a designated pressure and works as another rotatable surface moving body. The fixing roller and the pressure roller form a nip part. The toner image is fixed on the transfer medium by heat and pressure applied by using the nip part. The transfer medium where the toner image is fixed by the nip part is discharged via discharge path. On the other hand, a transfer medium not separated from the fixing roller or the pressure roller after passing through the nip part due to stiffness of a paper sheet or curvature of the fixing roller or the pressure roller is forcibly separated from the fixing roller or the pressure roller by a separation plate whose head end part is provided in the vicinity of the nip part so as to be discharged.
Generally, the transfer medium contains moisture. The moisture contained in the transfer medium is changed to vapor by heating the transfer medium by the nip part so as to be discharged from the transfer medium. The vapor discharged from the transfer medium is changed to condensation if coming in contact with a separation plate having a low temperature. Since the head end of the separation plate is provided in the vicinity of the nip part, the possibility of the head end part coming in contact with the transfer medium is higher than the probability of other parts of the separation plate coming in contact with the transfer medium. Because of this, if the condensation is generated at the head end part of the separation plate, the transfer medium may become adhered to the head end part of the separation plate so that a paper jam may be generated.
There are several suggestions to solve such a problem of condensation.
For example, a fixing device having a structure where a notch part is provided at a head end part of a separation plate so as to let vapor generated from a transfer medium go out is suggested in Japan Laid-Open Patent Application No. 2003-202767. Because of such a notch part for letting let vapor generated from the transfer medium go out, the condensation is prevented from being adhered at the head end part of the separation plate. However, in the fixing device discussed in Japan Laid-Open Patent Application No. 2003-202767, if vapor not discharged from the notch part for letting the vapor generated from the transfer medium go out is adhered at the head end part of the separation plate, condensation is generated at the head end part of the separation plate.
A fixing device having a structure where a separation plate is made of thin metal having low specific heat or a material having high heat conductivity, the separation plate is heated by contact of the transfer medium which is heated at high temperature, and the temperature of the separation plate rises to a temperature at which condensation is not generated, is suggested in Japan Laid-Open Patent Application No. 6-43772. In this fixing device, the condensation is prevented from being adhered at the head end part of the separation plate by heating the separation plate.
However, in the fixing device discussed in Japan Laid-Open Patent Application No. 6-43772, it takes a lot of time for the temperature of the head end part of the separation plate to rise to a temperature at which condensation is not generated. Hence, until the temperature of the head end part rises to the temperature at which condensation is not generated, the condensation is adhered to the head end part of the separation plate. Particularly, in a state where the fixing device is left as it is for a long time so that the fixing device is cooled, the temperature of the separation plate is low. Hence, in this case, even if the transfer medium heated at high temperature comes in contact with the separation plate, the temperature of the head end part may not have risen to the temperature at which condensation is not generated so that the condensation may be adhered to the head end part of the separation plate.
In addition, in the fixing device discussed in Japan Laid-Open Patent Application No. 6-43772, since the temperature of the entire separation plate rises evenly due to the heat of the transfer medium, it takes a lot of time for the temperature of the head end part of the separation plate to rise by the heat of the transfer medium to a temperature at which condensation is not generated.